In the LD or LD-AC conversion of pig iron into steel, oxygen is blown at high pressure directly down through a lance onto the bottom of the melt in the converter vessel. In addition a secondary oxygen stream afterburns the carbon monoxide generated by the refining process, creating harmless carbon dioxide and generating useful heat. The primary stream that is responsible for the conversion must be at very high pressure and typically exits from the lower end of the lance at supersonic speeds. The secondary stream that burns off the gaseous byproducts is at much lower pressure, typically exiting from the lower end of the lance at subsonic speeds, and annularly surrounds the primary stream.
Although both streams are of the same gas, pure oxygen, the pressures needed are so different that separate supply pipes are needed as well as complex remote pressure-regulating systems. Obviously this complicates the structure of the lance which itself is exposed to high temperatures and is otherwise in a very harsh environment.